1. Field of the Invention
The invention relates generally to switches used in communications networks. In particular, it relates to optical routers.
2. Background Art
The advancement of telecommunications technology over the past two decades has included two significant developments: (1) large, high-capacity networks based on packet switching; and, (2) optical fiber transmission media and in particular silica fiber and the use of wavelength division multiplexing to further increase the fiber bandwidth. Combining the two has presented some difficulties.
There are several types of commercially important packet networks. Asynchronous transfer mode (ATM) was developed in the telephone industry and is based on ATM cells having a fixed length of 53 bytes. Multiple ATM cells are identified to a virtual communications circuit.
Another type of network assuming greater importance and being implemented in many environments is related to the Internet communications network based on the TCP/IP protocol. The TCP/IP protocol applies to many levels of communications networks, but some of the most challenging applications involve the switched connections between different computer networks. An Internet-type of communications network 10, as schematically illustrated in FIG. 1, connects multiple terminals 12 through nodes 14 interconnected by bi-directional communications links 16. The terminals 12 can be considered to be ports to other, perhaps different, types of computer networks. The nodes 14 are based on routers which can route sequentially received frames in different directions as the frame propagates through the network 10 from the source terminal 12 to the destination terminal 12. The preferred term for packets is a frame 18, which as illustrated in FIG. 2, is composed for a serial link of a header 20 and an immediately following data payload 22. That is, the header 20 and payload 22 are time multiplexed. The header 20 contains among other items a destination for the frame 18. The data payload 22 is often of variable length, in which case the header 20 includes an indication of the length, but the overall frame is relatively short, on the order of a few hundred bytes. Sometimes a trailer is included to mark the end of the frame.
Although much of the following description is based on the multiply connected fiber network of FIG. 1 and with the routers being based at the nodes 14, the invention can be used with other types of networks, and routers are used in yet other configurations. In one example, the links may be of different forms linking different types of nodes, including satellites, airplanes, and complexly connected systems of multiple computers. In a second type of networks, as illustrated in the network diagram of FIG. 3, an inter-connected ring network 26 includes multiple bi-directional rings 28a, 28b, 28c, each including two counter-propagating optical fibers 30, 32, which provide redundant paths in case the pair of fibers 30, 32 is cut at any one point. That is, the rings 28 are survivable. Terminals 34 are connected to the respective rings 28 through nodes 36. Cross connects 38 link different ones of the rings 28. In a more realistic telephone or data network, a cross connect 38 may link more than two rings at a central communications hub.
Each of the rings 28 is typically controlled fairly tightly. A ring network which uses optical fiber for the transmission medium may employ wavelength division multiplexing (WDM), in which a single fiber conveys multiple optical carriers impressed with different data signals. In a WDM environment, packets between different pairs of terminals 34 on the same ring 28 may be identified and switched according to optical wavelength. However, such tight control becomes difficult for switching signals through the cross connects 38 between different rings 28. A packet switched system typically then requires that the cross connects 38 interrogate the frame header and switch only those frames destined to go outside of the originating ring 28. That is, the inter-ring cross-connects are advantageously based on routers. For a WDM environment, the cross-ring switching also strongly needs translation between WDM wavelengths to allow reuse of wavelengths and prevent undue constraints on routing and timing. It is also possible that the intra-ring nodes 36 are based on routers which extract from the ring 28 only those frames destined for the associated terminal 34. Further, the terminals 34 (or terminals 12 of FIG. 1) may represent an interface to a local network, such as an Ethernet network, in which only some of the packets need to be transferred from the local network onto the ring 28, for possible retransmission to yet other rings. Thus, the terminal 34 may additionally incorporate a router to transfer only selected ones of the packets that it receives from within the local network.
Returning to FIG. 1, the original TCP/IP networks were based on high-speed digital electrical links 16, on which the frames 18 are transmitted in sequential fashion. A router receives a frame 18 on an incoming link 16, determines from the header 20 where the frame 18 should go, and accordingly retransmits the entire frame 18 onto the desired outgoing link 16. Typically, the received frame is stored in a memory, called a buffer, which allows time for the router to determine from lookup tables which outgoing link corresponds to the destination address. The robustness of the Internet derives from the fact that the nodes 14 are nearly autonomous with very little central control and from the further fact that multiple paths usually exist between the source and destination terminals. In such a loosely controlled network, frames arrive at a node 14 at nearly random times with nearly arbitrary destinations. In particular, two or more frames may arrive nearly simultaneously on different incoming links and require switching to the same outgoing link. The buffer allows for temporary storage of frames awaiting retransmission on a busy link.
The data rate for a network based on electrical links is typically determined by the operating frequency of the electronic routers and their associated electronic receivers and transmitters. At present, the maximum data rate for commonly used electronic systems is about 10 gigabits per second (Gb/s) although 40 Gb/s systems are being developed. Further increases will prove difficult. A 10 Gb/s transmission link conveys a 500 byte frame (each byte being 8 bits) in 400 ns. Since packets need to be individually switched, packet switching times should be substantially less than this time in order to not impact transmission capacity.
Optical fiber presents many advantages in communications network including speed, cost, security, and noise immunity. As originally applied to networks, a fiber was used to carry in one of the fiber transmission bands a single optical signal that had been modulated by an electronic data signal. In a point-to-point system, each link of the network included optical receivers and transmitters including electro-optical (E/O converters at the respective nodes interconnected by an optical fiber. Three commonly used transmission bands extend over wavelengths in the neighborhoods of 850 nm, 1310 nm, and 1550 nm. The 1310 nm band is typically interpreted as extending from 1290 to 1330 nm, and the 1550 nm band extends from 1520 to 1580 nm. The 1310 nm band is usually used in local networks because of its low frequency dispersion and hence high data rates while the 1550 nm band is favored in long distance networks because of its lower absorption. The 850 nm band extending from 800 to 900 nm is also available for less extended networks. The wavelengths between the bands are generally not usable because of excessive fiber absorption. Efforts are continuing to expand the widths of these bands. Optical fibers of other compositions have other transmission bands, but non-silica fibers have little commercial importance at this time.
Although the intrinsic transmission bandwidth of silica fiber in any of these bands is vast, often measured in hundreds of terahertz, the system speed is limited by the speed of the electronics associated with the routers and its transmitters and receivers, that is, about 10 Gb/s. However, the network data rates can be significantly increased by the use of wavelength division multiplexing (WDM) in which a single fiber conveys multiple optical carriers of different optical wavelengths xcex1-xcexW in one or more of its transmission bands, and each wavelength carrier is impressed with a different data signal. An example of a WDM communication system includes forty or more wavelength channels (Wxe2x89xa740) with wavelength spacings at 1550 nm of about 1 nm or less. Thereby, the fiber transmission capacity is increased by a factor of forty.
The technology for the optical receivers and transmitters is readily available. Multiple semiconductor lasers of different emission wavelengths can be fabricated on one chip to have different emission wavelengths, the outputs of which are modulated according to the different respective data signals. An optical multiplexer combines the multiple optical signals and couples the multi-wavelength signal onto the fiber. At the receiver, the typically passive optical demultiplexer separates the different wavelength components, which respective optical detectors then convert to electrical form at data rates of no more than 10 Gb/s and ready for electronic switching. Optical multiplexers and demultiplexers are available that are essentially insensitive to data rate.
However, an electronically based router applied to the WDM fiber environment with E/O conversion between the fiber and the electronics does not scale well. Assume that K multi-wavelength router input ports are connected to K multi-wavelength router output ports and that there are W WDM wavelengths. For a system with 4 input ports and 32 wavelengths operating at 10 Gb/s, the total aggregate switching capacity needs to be 1.28 terabits per second (Tb/s). In the above design, each node or router requires KW electronically based optical transmitters and KW electronically based optical receivers. Further, a non-blocking electronic switch needs to connect any input port to any output port, for example, as implemented in a Clos switching network having multiple stages of parallel routers. As a result, its power and complexity increase as (KW)2. It has been estimated that an electronic router incorporating the current state of the art in the above 1.28 Tb/s switch would require 54 bays of electronics weighing over 4000 kg and consuming 400 kW of power, including more than 1200 thermoelectric coolers for the very temperature-sensitive optoelectronics.
Electronically based routers suffer the further disadvantage that the optical-to-electronic conversion and complementary electronic-to-optical conversion is based on a particular format, for example, either TCP/IP or ATM with a digital payload. There are applications in which it is desired to have different formats on different channels, for example, different digital format on different channels, or digital payloads on some channels and analog or mixed analog/digital payloads on other channels. Even within a single WDM channel, it is desirable that the format of different packets be freely chosen. While it is conceivable to design an electronic router with this flexibility, it is advantageous that the router be concerned only with the packet routing and not with the format of the payload. Insensitivity to the payload format allows proprietary formats to be used on a public network without knowledge at the router of the payload format. As a result, it is desirable that a router not decode the payload portion of a switched packet.
All-optical WDM communication networks have been proposed in which signals are switched at each of the nodes according to the wavelength of the optical carrier. Such a network is largely passive away from the terminals and can thus be made relatively small and inexpensively. The network is transparent from transmitter to receiver and is thus insensitive to protocol. However, while components are available which can reconfigure the wavelength connections, the reconfiguration typically requires hundreds of milliseconds and thus is clearly incompatible with the very short TCP/IP and ATM frames. Furthermore, the wavelengths need to be allocated for the system as a whole depending upon traffic. Such centralized control runs counter to the autonomous nature of the many nodes of the Internet.
I have described the use of wavelength conversion for WDM networks in xe2x80x9cWavelength conversion technologies for WDM network applications,xe2x80x9d Journal of Lightwave Technology, vol. 14, no. 5, June 1996, pp. 955-966. However, the article emphasizes transparent networks in which channels are switched according to wavelength and does not explore optical packet switching.
An optical router for wavelength-division multiplex (WDM) signals allows the switching of payloads without their conversion to electrical form. The headers are decoded and processed to determine the optical switching route.
A multi-wavelength optical router may be implemented by input and output array waveguide gratings (AWGs) acting as demultiplexers and multiplexers. The signals from the input AWG have their wavelengths converted to reflect a routing path to the outputs. Output wavelength converters may be used to readjust the wavelengths for transmission over the network.
A preferred embodiment includes a switching AWG positioned between the input and output AWGs. Input wavelength converters are positioned on single-wavelength channels between the input and switching waveguides and the degree of wavelength conversion is set by a control section reading the headers of arriving packets. Output wavelength converters are positioned between the switching AWG and the output AWG to convert the wavelengths for transmission. Multiple input and output AWGs with associated wavelength converters may be connected through a single switching AWG.
The AWGs and waveguides linking the optical element may be formed in a single substrate, for example of InP. Additional opto-electronics may also be included in the same substrate. Alternatively, a few substrates may be formed with the different stages of the router and mounted on a common substrate carrier providing optical coupling between the substrates. In either case, the router may be mounted on a single thermoelectric cooler.
The wavelength converters may be implemented as Mach-Zehnder interferometers with controllable active regions, preferably reversed biased into avalanche, and tunable lasers. Preferably the Mach-Zehnder interferometer and tunable lasers are fabricated in the same substrate as the AWGs and waveguides.
The size of the switching AWG may be reduced by a multiple levels of AWGs of lesser size. Two stages of such multi-level routers reduce contention.
The packet header information is preferably carried out-of-channel on an optical carrier of differing wavelength than any of the carriers of the packet payload. Headers for some or all of the WDM channels may be carried on one signaling carrier by the use of multiple RF subcarriers impressed on the signaling optical carrier. However, two or more signaling carrier wavelengths may be use.
The out-of-channel signaling carrier may be in the same fiber band as the data carriers, for example, the 1550 nm band, or may be in a different fiber band, for, example, the 1310 nm band. A simple demultiplexer such as a multi-mode interference filter can separate out-of-band signaling from the data carriers. The input AWG may be used to separate in-band, out-of-channel signaling from the data carriers.
Out-of-channel signaling may be sent in advance of the corresponding payload to allow time for header reception and processing to immediate switch the payload when it arrives.
Alternatively, the payload may be optically stored or delayed while the header is being processed. Preferably, optical delay elements are interposed on the wavelength-separated optical paths between the input AWGs and the input wavelength converters.
One type of fixed or selective and controllable optical delay is implemented by a waveguide structure including multiple quantum wells and a photonic bandgap arrangement of periodically arranged holes and isolated defects, thereby providing extended optical trapping at the defects by electromagnetically induced transparency. Such a structure may be formed in the same substrate as the rest of the router, but it may be used for other purposes than optical routing.
Photodetectors and laser diodes for the out-of-channel signaling may be formed in the same substrate, for example of InP, as the AWGs. High-speed receiver and transmitter electronics may be formed in separate chips, for example, of GaAs and bonded to the InP substrate. Multiple receiver and transmitter chips may be closely positioned to their respective opto-electronics. Slower control electronics may be formed in a silicon chip that is bonded also to the InP chip. The bonding is preferably performed with flip-side solder bumps that additionally provide electrical contacts, or wire bonding may alternatively be used.